High light extraction efficiency nitride based light emitting diode by surface roughening

ABSTRACT

A III-nitride light emitting diode (LED) and method of fabricating the same, wherein at least one surface of a semipolar or nonpolar plane of a III-nitride layer of the LED is textured, thereby forming a textured surface in order to increase light extraction. The texturing may be performed by plasma assisted chemical etching, photolithography followed by etching, or nano-imprinting followed by etching.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. Section 119(e) ofco-pending and commonly-assigned U.S. Provisional Patent ApplicationSer. No. 60/991,617, filed on Nov. 30, 2007, by Hong Zhong, AnuragTyagi, Kenneth J. Vampola, James S. Speck, Steven P. DenBaars, and ShujiNakamura, entitled “HIGH LIGHT EXTRACTION EFFICIENCY NITRIDE BASED LIGHTEMITTING DIODE BY SURFACE ROUGHENING,” attorneys' docket number30794.258-US-P1 (2008-277-1), which application is incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to light emitting diodes (LEDs) and moreparticularly to high light extraction efficiency gallium nitride basedLEDs via surface roughening.

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numberswithin brackets, e.g., [x]. A list of these different publicationsordered according to these reference numbers can be found below in thesection entitled “References.” Each of these publications isincorporated by reference herein.)

Gallium nitride (GaN) based wide band gap semiconductor LEDs have beenavailable for almost 15 years. The progress of LED development hasbrought about great changes in LED technology, with the realization offull-color LED displays, LED traffic signals, white LEDs, and so on.

High efficiency white LEDs have gained much interest as possiblereplacements for fluorescent lamps—the luminous efficacy of white LEDs(130-150 lumens/watt [1]) already surpasses that of ordinary fluorescentlamps (75 lumens/watt). Nevertheless, current commercially availablewurzite nitride based LEDs are characterized by the presence ofpolarization-related electric fields inside multi-quantum wells (MQWs),for their [0001] c-polar growth orientation. The discontinuities in bothspontaneous and piezoelectric polarization at the heterointerfacesresult in internal electric fields in quantum wells which cause carrierseparation (quantum confined Stark effect (QCSE)) and reduce theradiative recombination rate within quantum wells [2-5].

To decrease these polarization-related effects, growing III-nitridedevices on the nonpolar planes, viz, the (1-100) m-plane or the (11-20)a-plane, has been demonstrated [6-7]. Another approach to reduce, andpossibly eliminate those effects, is to grow III-nitride devices oncrystal planes that are inclined with respect to the c-direction, i.e.,semipolar planes. Devices grown on different semipolar planes, including(1-1-1), (10-1-3), (11-22) and others, have also been demonstrated[8-10]. These planes have reduced polarization discontinuity inheterostructures compared with the c-plane III-nitride materials; andfor semipolar planes oriented ˜45 degree from the c-plane, there is nopolarization discontinuity in InGaN/GaN heterostructures [5]. Recently,with the advent of high quality freestanding GaN substrates, highperformance nonpolar and semipolar LEDs with peak emission wavelengthsranging from 407 nm to 513 nm on nonpolar m-plane, semipolar (10-1-1),and (11-22) freestanding GaN substrates have been reported. Theperformance highlights of those LEDs are summarized in Table 1 [11-15].Those devices show greatly reduced polarization-related electric fieldsin the quantum wells, which enables one to employ thicker quantum wellsinside an LED, which is believed to be crucial for devices operatingunder high currents. Therefore, LEDs grown on nonpolar and semipolaroriented GaN substrates hold great promise for commercially usefulsolid-state lighting applications and could be commercially viable ashigh quality freestanding GaN substrates become more available.

TABLE 1 Summary of the performance of recently reported semipolar andnonpolar LEDs. External Quantum Efficiency at Peak Emission Output Powerat 20 mA drive Wavelength Crystal Orientation 20 mA drive currentcurrent 407 nm (violet-blue), Nonpolar m-plane, 23.7 mW, 20.58 mW 38.9%,33.9% 411 nm (violet-blue) Semipolar (10-1-1) plane 444 nm (blue)Semipolar (10-1-1) 16.21 mW (under 29% (under plane pulsed operations,pulsed operations, 10% duty cycle) 10% duty cycle) 489 nm (blue-green)Semipolar (11-22) 9 mW (under 18% (under plane pulsed operations, pulsedoperations, 10% duty cycle) 10% duty cycle) 516 nm (green) Semipolar(11-22) 5 mW 10.5% plane

Current techniques to improve the efficiency of an LED fall under twodistinct categories: increasing the internal quantum efficiency or theextraction efficiency.

Increasing the internal quantum efficiency, determined by crystalquality and epitaxial layer structure, could be rather difficult. Atypical internal quantum efficiency value for blue LEDs is more than 70%[16] and an ultraviolet (UV) LED grown on a low-dislocation GaNsubstrate has recently exhibited an internal quantum efficiency as highas 80% [17]. There might be little room for improvement over thesevalues, especially for nonpolar and semipolar oriented devices grown onhigh quality freestanding GaN substrates.

On the other hand, there is plenty of room for improving the lightextraction efficiency. For a bare chip nitride based LED, because of therather huge difference between the refractive indices of GaN (n=2.5) andair (n=1), the angle of the light escape cone is only 23 degrees, whichleads to a meager light extraction efficiency that is as low as 4.18%[18]. The light outside the escape cone is reflected repeatedly insidethe device and eventually absorbed by the active region or theelectrodes.

Surface roughening procedures could be used to significantly reduceinternal loss of light and encourage light escape from the device. FIG.1 is a schematic cross-sectional illustration of a surface roughenedLED, comprising an n-type electrode 10, n-type III-nitride layer 11,III-nitride active region 12, p-type III-nitride layer 13 and p-typeelectrode 14 that is bonded to a silicon sub-mount 16 via a gold tinbonding 15. A photo-enhanced chemical (PEC) etching is used to roughenthe backside 17 of the n-type layer 11, which is a nitrogen-face(N-face) GaN surface. The arrow 18 indicates a possible trajectory forlight emitted by the LED. A 130% increase in output power was measuredfor a surface roughened LED compared with a smooth surface and otherwiseidentical device [19].

Although surface roughening by PEC etching is a sine qua non forimproving light extraction from a nitride based LED, the effectivenessof this technique by and large hinges on the crystal orientation andpolarity of the to-be-roughened surface, particularly, the nitrogen faceof a c-polar [0001] GaN [21]. As a result, PEC etching could not beapplied to surfaces of other GaN crystal orientations and polarity,including a-face (11-20), nonpolar m-face (1-100), and most of thesemipolar surfaces. The lack of means for surface roughening has becomea major hurdle for nonpolar and semipolar LEDs to achieve higherextraction efficiency and hence higher overall efficiency, and thereforeimproved roughening techniques are needed to address this issue.

SUMMARY OF THE INVENTION

The present invention describes a method of increasing the lightextraction efficiency from a nitride based LED, which involvesphotolithography and plasma-assisted chemical dry etching. Throughincreasing light extraction, subsequent improvement of efficiency isthus expected. One most noticeable advantage of the present invention isthat it significantly increases the light extraction efficiency from anitride-based LED, including films that are grown along nonpolar andsemipolar orientations. In addition, this invention is morestraightforward compared to other light extraction enhancementtechniques, such as using a photonic crystal. More important, unlikephoto-enhanced chemical etching that is also a simple light extractionenhancement technique, the present invention is more versatile as itcould be applied to any nitride semiconductor surface regardless of itscrystal structure.

Therefore, to overcome the limitations in the prior art described above,and to overcome other limitations that will become apparent upon readingand understanding the present specification, the present inventiondescribes a method for fabricating a III-nitride LED, comprisingtexturing at least one surface of a semipolar or nonpolar plane of aIII-nitride layer of the LED to form a textured surface, wherein thetexturing step is performed by plasma assisted chemical etching. Thetexturing step may be performed by photolithography followed by theetching, or the textured surface may be formed using nano-imprintingfollowed by the etching. Light emitted by an active region of the LED ismostly extracted from the textured surface.

The texturing step may further comprise: (1) creating at least onefeature with at least one sidewall that reflects and transmits at leastone light ray incident from inside the feature; and inclining thesidewall such that each time the ray is reflected, an angle of incidenceof the ray relative to a surface normal of the sidewall decreases, suchthat when the angle of incidence of the ray is smaller than a criticalangle, the ray's transmission through the sidewall is increased, andwhen the angle of incidence of the ray is at least equal to the criticalangle, the ray is reflected by the sidewall.

The present invention further discloses a method for emitting light froma III-nitride LED, comprising emitting the light from at least onetextured surface of a semipolar or nonpolar plane of a III-nitride layerof the LED, wherein the texturing is performed by plasma assistedchemical etching.

The present invention further discloses a III-nitride LED, comprisingn-type III-nitride; p-type III-nitride; a III-nitride active layer, thatemits light, formed between the n-type III-nitride and p-typeIII-nitride; a III-nitride light extraction surface on the n-typeIII-nitride and forming an interface with an external medium, whereinthe III-nitride light extraction surface has features with at least onesloped sidewall which transmits the light into external medium air atthe interface and reflects the light at the interface, wherein: (1) thereflected light, after undergoing subsequent reflections inside thefeatures, has an increased incidence angle relative to the interface andconsequently an increased chance of being transmitted to the externalmedium, and (2) the n-type III-nitride, p-type III-nitride, andIII-nitride active layer are semi-polar or non-polar layers. Theexternal medium may be a medium with a smaller refractive index thanIII-nitride, for example air or a vacuum.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a schematic cross-section of an (Al, Ga, In)N LED withbackside roughened by photo-enhanced chemical etching.

FIG. 2 is a schematic cross-section of an (Al, Ga, In)N LED in asuspended geometry with backside roughened by the current invention.

FIG. 3 a is a scanning electron microscopy (SEM) image of a GaN surfaceafter roughening.

FIG. 3 b is a cross-sectional SEM image of a GaN surface afterroughening.

FIG. 4 illustrates the process of light escaping from a conical feature.

FIG. 5 is an optical microscopy image of a GaN surface after roughening.

FIG. 6 illustrates the geometries of a conical feature.

FIG. 7 is a flowchart, illustrating a method for fabricating a highlight extraction efficiency LED structure.

FIG. 8 is an SEM image of a semipolar (11-22) GaN surface afterroughening by photo-enhanced chemical etching.

FIG. 9 is a schematic cross-section of an (Al, Ga, In)N LED in aflip-chip design with backside roughened by the current invention.

FIG. 10 is a schematic cross-section of an (Al, Ga, In)N LED in aflip-chip design with backside roughened by the current invention, andp-type III-nitride layer bonded with shaped zinc oxide or a transparentconductor.

FIG. 11 is a flowchart illustrating a method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

TECHNICAL DESCRIPTION

The present invention describes a technique for increasing the lightextraction efficiency from a nitride based LED, which involvesphotolithography and plasma-assisted chemical dry etching. Throughincreasing light extraction, subsequent improvement of efficiency isthus expected.

In one embodiment of the present invention, the freestanding GaNsubstrate surface of the opposite side of the LED growth front isroughened. After the device is fabricated, the LED is then placed withina shaped optical element.

FIG. 2 shows a schematic representation of a surface roughened LEDpackaged in a suspended geometry according to the preferred embodimentof the present invention. The LED is comprised of a p-type metalelectrode 20, semi-transparent p-type electrode 21, p-type III-nitridelayer 22, III-nitride active region 23, n-type III-nitride layer 24,double-side-polished freestanding GaN substrate 25 on which surfaceroughening is performed by plasma-assisted chemical etching, metalheader 26, metal wire 27 (connecting to the p-electrode 20), metal wire28 (connecting to the n-type metal electrode 29), and a silicone conemold 30 into which the LED chip encapsulates. The arrows 31 indicatepossible trajectories for light emitted by the LED.

Plasma-assisted chemical etching is performed on the backside surface 32of the GaN substrate 25 after etching masks are defined. With a certainproportion of different corrosive gases, including, but not limited to,chlorine and fluorine based gases, and other gases, under certainchamber pressure and plasma powers, plasma-assisted chemical etchingdefines the unmasked area with features that are characterized by slopedsidewalls. As a result, the surface 32 is roughened through theformation of conical features that tile up most of the surface 32.Concavity that resembles a meteorite crater might be formed on top ofeach conical feature as a result of a combination of appropriate etchingconditions, etching time, and the use of material of the etching masks.Etching conditions of an exemplary roughening procedure, performed by aninductively coupled plasma (ICP) etcher, include a certain ratio ofchlorine-based and fluorine-based gases (10:1 to 150:1), appropriate ICPpower, ranging from 100 Watt to 1000 Watt, desirable bias power, rangingfrom 10 Watt to 500 Watt, and suitable chamber pressure (1-50 Pascal).

FIG. 3 a is an SEM image of a GaN surface after a 30 minute treatment ofthe exemplary roughening procedure with the use of circular etchingmasks (2 micron in diameter and 8 micron apart from center to center),and FIG. 3 b is a cross-sectional SEM image of the same sample. FIGS. 3a and 3 b illustrate how the surface is roughened through the formationof conical features 33 (with sloped sidewalls 34) that tile up most ofthe surface, and how a concavity 35 might be formed on top of eachconical feature 33.

Conical features (or truncated conical features) are believed to bebeneficial to light extraction [20]. FIG. 4 illustrates the process oflight escaping from such a conical feature 40, 33. Light rays 41incident at the feature-air boundary 42 are either transmitted throughthe nitride semiconductor-air interface (or feature-air boundary 42)(dashed arrows 43), or reflected by the boundary 42 (solid arrows 44).Most of the reflected rays 44, after undergoing subsequent reflections45 inside the feature 40 that leads to an increase of incidence angle(90°-θ) at the semiconductor-air interface (or feature-air interface42), can eventually escape 46 from the cone 40 by a near normalincidence at the feature-air interface 42. In the above, the feature-airboundary 42 and semiconductor-air interface are equivalent. Light rays41 originate from the active region, for example, and the feature 40typically has an interface 47 with an n-type layer of the nitride.

FIG. 5 is an optical microscopy image of a roughened surface. Thediscolored and dark surface, resulting from the roughening procedure,can be ascribed to the light scattering at the boundary of air and GaN,and such surface in general has better light extraction characteristicsthan a smooth and “mirror-like” counterpart [19].

Geometries of the roughened features 60, as shown in FIG. 6, includingthe sidewall inclination angle 61, cone diameters 62 and 63, and theheight of the cone 64, as well as the number density of those conicalfeatures 60, could be tailored for optimal light extraction with the useof appropriate etching masks and suitable etching conditions. It shouldbe noted that plasma assisted chemical etching is a non-equilibriumprocess, therefore this roughening procedure can be applied on anynitride semiconductor surfaces, regardless of their crystal orientationsand polarity.

With such a design, light generated within the active region is able toescape effectively from both sides of the die; and the extractionefficiency of the light propagating toward the substrate can beconsiderably increased because of the surface roughening. As a result,an improvement over the output power is expected.

Processing Steps

FIG. 7 illustrates the processing steps for one embodiment of theinvention.

Block 70 represents the step of growing epitaxial layers (devicegrowth), for example on a double-side-polished freestanding GaNsubstrate by Metal-Organic Chemical Vapour Deposition (MOCVD), therebycreating the sample.

Block 71 represents the step of annealing the sample for activation ofthe p-type dopants (p-type activation).

Block 72 represents the step of surface roughening throughplasma-assisted chemical etching.

Block 73 represents the step of cleaning the roughened sample usingsolvents and acids (sample cleaning).

Block 74 represents the step of depositing a p-type electrode (on thep-type layer), for example, a nickel and indium tin oxide (ITO)semi-transparent electrode.

Block 75 represents the step of defining the mesa areas by chlorinebased dry etching, for example.

Block 76 represents the step of depositing p-type and n-type metal pads,for example deposition of the titanium, aluminum, nickel and gold n-typeelectrode and p-type electrodes.

Block 77 represents the step of packaging the LED, for example in asuspended geometry.

Possible Modifications and Variations

The LED may be comprised of polar c-face (0001) (Al, Ga, In)N, non polara-face (11-20) and m-face (1-100) (Al, Ga, In)N, or semipolar (Al, Ga,In)N, wherein semipolar refers to a wide variety of planes that possestwo non-zero h, i, or k Miller indices, and a non-zero 1 Miller index,{hikl}.

Moreover, besides freestanding and bulk GaN substrates, the LED may begrown on a foreign substrate, for instance, a sapphire, silicon carbide,silicon, germanium, gallium arsenide, gallium phosphide, indiumphosphide, or spinel wafer, and techniques, such as laser lift-off, canbe employed to separate the substrate and the nitride semiconductor sothat the roughening process can proceed.

If the crystal orientation of the to-be-roughened surface is a semipolar(11-22) oriented GaN surface, surface roughening could be also performedby the photo-enhanced chemical (PEC) etching procedure. The roughenedsurface is covered by one or more triangular pyramids that are comprisedof a c-polar (0001) GaN surface and m-face [1-100] GaN surfaces, asshown in FIG. 8.

This roughening technique could be applied to various high lightextraction efficiency LED structures, other than the one covered in thepreferred embodiment.

FIG. 9 is a schematic representation of a high light extractionefficiency LED according to an exemplary embodiment of the presentinvention. The LED, in a flip-chip structure, comprises a freestandingGaN substrate 91 roughened 92 by the invention, n-type III-nitride 93,n-type electrode 94, III-nitride active region 95, p-type III-nitridelayer 96, p-type electrode and light reflector 97, and host sub-mount98. The arrow 99 indicates a possible trajectory for light emitted bythe LED.

FIG. 10 is a schematic representation of a high light extractionefficiency LED, according to an exemplary embodiment of the presentinvention. The LED comprises an n-type III-nitride layer 1001,III-nitride active region 1002, p-type III-nitride layer 1003, n-typeelectrode 1004, and a host sub-mount 1005. The backside 1006 of thefreestanding GaN substrate 1007 is roughened by the present invention.Adjacent the p-type III-nitride layer 1003 is an n-type zinc oxide (ZnO)cone-shaped element 1008 with a p-type electrode, which can help improvelight extraction of light 1009 a emitted by the active layer 1002 towardthe p-type layer 1003. The arrows 1009 a, 1009 b indicate possibletrajectories for light emitted from the active region 1002 of the LED.The external medium 1010 is also shown.

FIG. 11 is a flowchart illustrating a method for fabricating aIII-nitride light emitting diode (LED).

Block 1100 represents the step of texturing at least one surface of asemipolar or nonpolar plane of a III-nitride layer of the LED to form atextured surface 1006, wherein the texturing step is performed by plasmaassisted chemical etching. The texturing step may be performed byphotolithography followed by the etching. The textured surface 1006 maybe formed using nano-imprinting followed by the etching. Light emittedby an active region of the LED may be mostly extracted from the texturedsurface 1006. The texturing step of Block 1100 may further comprise(referring also to FIG. 4):

(1) Block 1101, representing the step of creating at least one feature40 with at least one sidewall 42 that reflects 44 and transmits 43, 46at least one light ray 41 incident from inside the feature 40, and

(2) Block 1102, representing the step of inclining the sidewall 42 suchthat each time the ray is reflected 44, an angle of incidence θ of theray 44 to a surface normal n of the sidewall 42 decreases, such that (a)when the angle of incidence θ of the ray is smaller than the criticalangle (θ_(c)), the ray's transmission 46 through the sidewall 42 isincreased, and (b) when the angle of incidence θ of the ray 41, 44 is atleast equal to the θ_(c), the ray is at least partially reflected 44 bythe sidewall 42. θ_(c) is the critical angle above which total internalreflection occurs, θ_(c)=arcsin (n_(external)/n_(internal)), whereinn_(ext) is the refractive index of the external medium 1015 andn_(internal) is the refractive index of the internal medium, i.e. thefeature 40. The surface normal n is an imaginary line perpendicular tothe sidewall 42.

Block 1103 (referring also to FIG. 4 and FIG. 10) represents a devicefabricated using the method of FIG. 11. The device may be a III-nitridelight emitting diode (LED), comprising n-type III-nitride 1001; p-typeIII-nitride 1003; a III-nitride active layer 1002 for emitting light(1009 a, 1009 b), formed between the n-type III-nitride 1001 and p-typeIII-nitride 1003; a III-nitride light extraction surface 1006 on thesubstrate 1007 or on the n-type III-nitride 1001 and forming aninterface with an external medium 1010, wherein the III-nitride lightextraction surface 1006 has features 40 with at least one slopedsidewall 42 which transmits the light 1009 b into the external medium1010 at the interface and reflects the light at the interface, wherein:(1) the reflected light 44, after undergoing subsequent reflections 45inside the features 40, has an increased incidence angle (90°-θ) at theinterface 42 and consequently an increased chance of being transmitted1009 b to the external medium 1010, and (2) the n-type III-nitride 1001,p-type III-nitride 1003, and III-nitride active layer 1002 aresemi-polar or non-polar layers. The external medium 1010 is typically amedium with a smaller refractive index than III-nitride, for example airor a vacuum.

In the above description, III-nitrides may be referred to as Group IIInitrides, or just nitrides, or by (Al, Ga, In, B)N, or byAl_((1-x-y))In_(y)Ga_(x)N where 0<x<1 and 0<y<1.

ADVANTAGES AND IMPROVEMENTS

One most noticeable advantage of the present invention is that itsignificantly increases the light extraction efficiency from anitride-based LED, including LEDs that are grown along nonpolar andsemipolar orientations. In addition, this invention is morestraightforward compared to other light extraction enhancementtechniques, such as using a photonic crystal. More importantly, unlikePEC etching, which is also a simple light extraction enhancementtechnique, this invention is more versatile as it could be applied toany nitride semiconductor surface(s) regardless of their crystalstructures. The present invention will enable high power and highefficiency LEDs.

REFERENCES

The following references are incorporated by reference herein.

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CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

1. A method for fabricating a III-nitride light emitting diode (LED), comprising: texturing at least one surface of a semipolar or nonpolar plane of a III-nitride layer of the LED to form a textured surface, wherein the texturing step is performed by plasma assisted chemical etching.
 2. The method of claim 1, wherein the texturing step is performed by photolithography followed by the etching.
 3. The method of claim 1, wherein the textured surface is formed using nano-imprinting followed by the etching.
 4. The method of claim 1, wherein light emitted by an active region of the LED is extracted from the textured surface.
 5. The method of claim 1, wherein the texturing step further comprises: (1) creating at least one feature with at least one sidewall that reflects and transmits at least one light ray incident from inside the feature; and (2) inclining the sidewall such that each time the ray is reflected, an angle of incidence of the ray relative to a surface normal of the sidewall decreases, such that: (a) when the angle of incidence of the ray is smaller than a critical angle, the ray's transmission through the sidewall is increased, and (b) when the angle of incidence of the ray is at least equal to the critical angle, the ray is reflected by the sidewall.
 6. A method for emitting light from a III-nitride light emitting diode (LED), comprising: emitting the light from at least one textured surface of a semipolar or nonpolar plane of a III-nitride layer of the LED, wherein the texturing is performed by plasma assisted chemical etching.
 7. A III-nitride light emitting diode (LED), comprising: (a) n-type III-nitride; (b) p-type III-nitride; (c) a III-nitride active layer, that emits light, formed between the n-type III-nitride and p-type III-nitride; (d) a III-nitride light extraction surface on the n-type III-nitride and forming an interface with an external medium, wherein the III-nitride light extraction surface has features with at least one sloped sidewall which transmits the light into external medium air at the interface and reflects the light at the interface, wherein: (1) the reflected light, after undergoing subsequent reflections inside the features, has an increased incidence angle relative to the interface and consequently an increased chance of being transmitted to the external medium, and (2) the n-type III-nitride, p-type III-nitride, and III-nitride active layer are semi-polar or non-polar layers.
 8. The LED of claim 7, wherein the external medium is a medium with a smaller refractive index than III-nitride.
 9. The LED of claim 7, wherein the external medium is air or a vacuum. 